1. Technical Field
The present invention relates to method for imparting colors and to control tone/shade of the imparted colors to colorless gemstones or decorative objects to obtain a desired shade of colors in the crystals. More specifically, the present invention relates to methods wherein tone of the imparted colors in colorless gems or crystals can be controlled. Even more specifically, the present invention relates to radiation-free, environment-friendly methods by which colored gemstones/crystals of desired color-tone can be produced.
2. Description of the Related Art
A variety of colored crystals are used in ornaments and decorative items. In recent years, the gems studded apparels are also becoming popular. Thus among minerals, the gem-crystals constitute an important class of mineral. About 2000 kinds of gem-minerals are available in nature, out of which around 100 are most popular. The cost and market demands of such minerals largely depend on their properties such as color, shine, transparency etc. and their over all appearance. The mother-earth provides both colored and colorless minerals. However, proportionately, colored crystals of the natural minerals such as gemstones are not available in large quantities compared to colorless ones. Therefore, the market demand of colored crystals is not met by the natural re-sources. Further, most of the colored gem minerals provided by the nature do not have aesthetically pleasing appearance. Therefore, a number of techniques have been invented to enhance their properties and to impart pleasing colors in colorless crystals.
Based on the scientific knowledge of color causing phenomena in natural gem crystals, a variety of techniques have been developed in the art to enhance the aesthetic properties of the colorless or paler gem minerals. The techniques such as electron, neutron, cobalt-60 irradiation, heat treatments, coating of multi layer, diffusion etc. have been developed to impart colors to transparent crystals particularly, colorless gem minerals.
For irradiation, Gamma reactors or Linear accelerators are employed to induce colors in colorless gems. U.S. Pat. No. 5,477,055, U.S. Pat. No. 4,749,869, U.S. Pat. No. 2,945,793 and many more describe different techniques to produce radiation treated colored gems. For intense color or for inducing colors in large size crystals a high dose with very high energy gamma rays are needed (U.S. Pat. No. 5,084,909). Radiation technique is widely used to produce colored gemstones for the past 50 years or so.
U.S. Pat. No. 3,490,250 describes a technique of multi layer coating of refractive materials on colorless gems. Starcke et al. (U.S. Pat. No. 5,853,826) also claimed a similar coating technique to improve colors of transparent materials. The coated crystals appear colored due to optical interference effect caused by reflection of light at interfaces of the deposited layers. A Japanese Patent JP60119506A2 also describes this method to produce color filters. To produce colored appearance in gemstones a technique based on joining a colored stone to a colorless gem has been described in Meisnner's patent U.S. Pat. No. 6,025,060. This patent teaches to produce colored composite gemstones. This approach has been followed in Signity America Ltd patent (WO 2007/123600 A1) to produce multiplet gemstones wherein a translucent printed image is embedded in between flat surfaces of two stones to obtain a composite, esthetically pleasant gem for use in jewelry. Thus instead of a colored piece as used in Meissner's technique, a custom image is inserted between two stones to produce a composite gem in Signity invention.
In recent past a technique based on diffusion evolved to impart colors to gem stones. Though the phenomenon of diffusion was known much earlier, however, the technique matured only after invention of semiconductor transistor in 1947. Semiconductor technologists developed a number of methods such as thermal diffusion, chemical vapour deposition (CVD), physical vapour deposition (PVD), ion-implantation etc. to accomplish diffusion in semiconductors to fabricate integrated circuit chips. Carr et al. (U.S. Pat. No. 3,897,529 are probably, the first ones to apply diffusion technique to enhance gems. Their subsequent patents U.S. Pat. No. 3,950,596 and U.S. Pat. No. 4,039,726, claim production of red, pink and blue colors in corundum by heating in powder of oxides of chromium, titanium, ferric and aluminium. Japanese Pat. No. 115998 and U.S. Pat. No. 2,690,630 also describe powder process to impart blue color in sapphire. Subsequently, it was recognized that diffusion of cobalt in sapphire is the cause of its blue coloration (Kane et al. Gems & Gemology, Summer 1990, 115-133). However, Shanon reported cobalt in blue colored minerals as early as in 1923 (American Mineralogist, vol. 8, No. 4, pp 147-148). In this technique colorless crystals in contact with powder of a color-causing reagent are heated at a temperature above 1300 degree Celsius to produce colors in them. Richard Pollak also applied the powder technique of Carr et. al to enhance color of topaz, quartz, garnet and chrysoberyl by heating them in fine powder of cobalt and cobalt oxide. U.S. Pat. No. 5,888,918 of Pollak employs a treatment temperature in the range of 900 degree Celsius to 1250 degree Celsius, which he further improved to 825 degree Celsius to 1050 degree Celsius according to his U.S. Pat. No. 6,376,031, U.S. Patent Appl. No. 20020174682 and WO 98/48944. However, in all these inventions a long treatment time between 3 hours to 200 hours is employed. According to U.S. Patent Appl. No. 20020128145, Pollak succeeded further in his efforts to reduce the treatment temperature range to 700 degree Celsius up to about 1000 degree Celsius but for a time period in the range of about 3 hours up to about 600 hours. The increase in treatment time according to this patent is understandable as those of skill in the art recognize that at low temperatures, long diffusion times are required to accomplish a process.
In a recent patent Gupta et al. (U.S. Pat. No. 6,872,422 B2) invented a technique to impart colors to gem minerals by diffusion. This patent employs coating of thin film of a colorant reagent followed by a heat treatment to produce colored gems. The technique provides better control on color intensity and significant improvement in treatment times to produce diffused colored gems. This thin film process claims a number of advantages over the powder methods.
Recently, Swarovski & Co. patent, (AT 411 464 B, 26, Jan. 2004) and Rauch et. al. patent (U.S. Pat. No. 7,033,640 B2, April 2006) report a technique wherein instead of use of powders of metal and metal oxides of Pollak's inventions discussed above, a sieve plate primarily made of cobalt and cobalt oxide (or iron oxide or vanadium oxide) with Aluminum oxide as an additive, has been employed to impart colors to gems. Their US patent, U.S. Pat. No. 7,033,640 B2 is English version of Oesterreich patent AT 411 464 B. However, the hitherto known processes of the mentioned inventions suffer from a number of drawbacks. Irradiation methods are limited in terms of cost, safety, efficacy and the like. Further, the radiated crystals are likely to loose their color in case they encounter exposure to a high temperature in their use. Though irradiation is a widely used technique for gems coloring and enhancement but one major concern of the radiation process is that the irradiated crystals (also named as “Nuked Stones”) stay radio-active for up to a year or more. Geiger Mueller Counter is used to measure the reminiscent radiations in the irradiated gems. U.S. safe limit of reminiscent radiation is 1 nano-curie/gm and that of Asian is 2 nano-curie/gm. A 50 nano-curie emission from the stones is believed to cause skin cancer and destroy white blood cells.
A major disadvantage of multi layers coating technique is that this does not induce any color into the crystals. The crystals appear colored because of reflection and interference of light by the layers deposited on surface of the crystal. Further, exposure of these crystals to high temperature or to acids treatment during their use in ornament making or any other such use the coated crystals are likely to result in loss or change of their colors.
The techniques to produce composite gemstones as reported in U.S. Pat. No. 6,025,060 gives a product, which only appears colored, or a gem containing an image. This technique is also labor intensive and is not time effective.
As readily recognized by those of skill in the art of diffusion, that diffusion constant of an element is a strong function of temperature. Therefore, under generally recognized diffusion principles, one would not expect invention methods to work during the average human life at the moderate temperature. This can obviously be noted in Pollak's U.S. Patent Application No. 20020128145, that the reduction in diffusion temperature to 700 degree Celsius in this invention, the diffusion time has increased enormously to 600 hours. Such a long exposure to heat is not only expensive but also likely to cause damage to crystals body/surface. Further, such a long treatment time is similar to long cooling periods required for radiation treated gems and is one of the vital concerns for commercial production. The other major drawbacks associated with powder diffusion process are: additional thermal load of powder/s to the furnace; uneven coloration and color patches on the surface of the treated stones; occurrence of surface damages due to sticking of powder particles at high temperature; two step heating; acid cleaning and/or polishing after heat treatment to remove powder particles that get adhered to the treated stones; risk of surface damage in post cleaning of treated stones and safety precautions against handling of acids and nano sized powders. In a powder process, stones are buried in a powder of coloring reagent. Also to control color intensity, powder/s of diluting agent such as oxides of magnesium, aluminium etc. is also mixed with the colorant. Since an extremely small quantity of colorant is used for imparting color to stones, the diluting powder/s adds to thermal mass and take up a very great deal of furnace space. The uneven coloration or color patches in powder process is inherent as the surface of a crystal buried in colorant powder is not uniformly in contact with the powder particles. Further, the finite size (even powder particles are of nanometer in size) of powder particles inherently, result in some finite gap between two adjacent particles contacting the crystal surface. At such gaps the diffusion of colorant atoms is much less compared to that at contacting points on the stone surface. This therefore, leads to a non-uniform diffusion and thereby uneven coloration or color patches in the finished products. It is very well realized by those of expert in art that at high temperatures the powder particles stick a solid surface of the crystals and therefore, some cleaning or polishing treatment is always needed to regain the shine of the treated gems. Other economic concerns in Pollack's powder process are: two steps of heating (first with colorant powder and second with neutral powder), and cleaning of individual stones after heating with powder. All these drawbacks make the powder methods expensive and time consuming. The most important technical shortcoming of a powder process is that there is no intimate contact between the crystal and atoms of diffusing reagent. This necessitates either a higher temperature or a longer time for diffusion of atoms from the power into the host crystal. Perhaps this is why hundreds of heating hours are employed in Pollak's methods. Further, safety precautions to handle acids and particularly, finely divided particles are also other limitations of the powder processes. Threats posed by recently developed nano-materials to human health and environment has now become a major global concern.
Ref.(http:/news.nanoapex.com/modules.php?name=News&file=article&sid=3592) is offering research projects to study safety aspects for handling nano-material powders. The similar drawbacks of powder process have also been highlighted in the Rauch et al. patent U.S. Pat. No. 7,033,640 B2.
The Rauch et al. patent is a powder process as for preparation of a sieve plate powders are used. Also during conditioning of the sieve plate, a neutral powder is used to prepare the protective layer. Preparation of stone holding plate i.e. the sieve plate is a ceramic technology as metallic oxides are involved in this During heat treatment for conditioning of the plate in this method, cobalt atoms diffuse into the protective layer, which subsequently acts as a diffusion source for coloring the gemstones. Thus during treatment the colorant atoms are indeed in the vicinity of polished surface of the stones. The most important draw back of the process is that it is specific to gemstone shape and size. We know that in gem industries an enormous number of sizes and shapes are involved. To impart colors to all types of gems one has to prepare a large number of such sieve plates. Even if largest sized recesses are made in a plate to also treat smaller size stones, different plates are required for different shapes of gems. Further, the large size holes and spacing between them occupies more furnace space. The large sieve sized plates also reduce the number of stones for treatment per unit space of the furnace. It is evident from the claimed method that only the diffusion process is responsible for induction of colors in gems placed on the plate. It is well known through semiconductor technology that for diffusion to occur efficiently, the diffusing impurity has to be available in atomic form. And for this reason all diffusion processes in established semiconductor technology use impurities either in liquid or gaseous state or a solid source having high vapour pressure at a diffusion temperature. Metal/metal oxide plate used in Rauch et al process act as source of diffusing atoms for the stones in contact with the sieve body or the protective layer. Therefore, like in a powder process, in this method also there is no intimate contact between the gemstone surface and the coloring material. In this case also, the finite size of aluminium oxide powder used on the plate to provide a protective layer introduces a finite gap between the diffusing source and stones. This therefore is likely to lead to high treatment temperature or longer periods to diffusion to occur into the stones. In brief, this process involves cumbersome ceramic technology for preparation of sieve plates and incorporates most of the drawbacks of powder methods. Thin film based processes are backbone of modern semiconductor industries because these are highly reliable and reproducible. It is not appropriate to compare thin film based methods to ceramic technology at least in term of cleanliness and contamination.
Gupta et al. (U.S. Pat. No. 6,872,422 B2) invention uses colorant reagent in thin film form thus this does not add to the furnace load, thin film makes intimate contact to the crystal surface for efficient diffusion of coloring atoms in the crystal body, color intensity is controlled by thickness of the deposited film, no post cleaning is needed as all colorant material is consumed in imparting color to the stones and only one heating cycle for diffusion of the colorant atoms into gemstone is required. However, treatment cycle (heating time and temperature) of Gupta et al. process also employs temperature as high as 1200 degree Celsius and diffusion time as large as 10 hours.
When solid crystals are subjected to high temperatures and for a long duration at a particular temperature, they are likely to get damaged or break due to thermal effects. In powder processes of gem enhancement, Carr et al. (U.S. Pat. No. 3,897,529) employed 1750 degree Celsius. Pollak's invention succeeded in achieving the treatment temperature in the range of 700 degree Celsius to 1000 degree Celsius according to U. S. Pat. No. 20020128145. However, the treatment time increased form about 100 hours of Carr et al. methods to 600 hours in Pollak's processes. Gupta et al. (U.S. Pat. No. 6,872,422 B2) claimed gems enhancement at a temperature in the range of 700 to 1200 degree Celsius for a treatment time in the range between 30 minutes to 10 hours.
For the past 8 years our research is particularly focused on this aspect of the crystals coloration and the present invention is the result of that. We know that according to general theory of diffusion in solids, there is a limit on reduction of diffusion temperature as diffusion constant of an element is strongly temperature dependent. So if the temperature is reduced below a particular limit, the general theory of diffusion predicts that the diffusion process take very long time to get completed. Therefore, from practical point of view particularly, for production purpose a technology employing very low diffusion temperature is not advisable. For example, 600 hours of a treatment cycle according to U. S. Pat. No. 20020128145 needs 25 days to complete a treatment cycle and this may not be suitable to a production platform.
In Pollak's processes there are two major aspects that govern the treatment cycle. First, despite the use of finally divided powder (U. S. Pat. Appl. No. 20020174682 A1) there is always a finite gap between particles and also between particles and the stone surface. Since the in-diffusing atoms of the powder material require to travel this gap before they actually diffuse into the crystal, the long diffusion time is therefore necessary for completion of the treatment in a powder based diffusion process. Second important aspect that has been overlooked in the invented powder processes is related to use of compound/oxide as colorant reagent. The diffusion is an atomic process and for a diffusion to occur the compound/oxide molecules need to be broken into atoms to initiate the diffusion. The general chemical principles suggest that breaking of bonds require some specific amount of energy and thus more thermal energy is needed in case a compound/oxide is used as a diffusion source. Therefore, high temperatures or long diffusion times are inevitable in powder processes technology. In Gupta et al. processes, two or more layers (one over the other) of coloring materials are employed to produce different colors in gems. So even if a coloring material is not a compound, atoms of second layer material have to either travel through the thickness of first layer or have to wait till first layer is completely consumed in the crystal. This necessitates employment of longer diffusion time compared to a single layer process to accomplish the process.
Hence there is need to develop a method for imparting colors and to control tone/shade of the imparted colors to colorless gemstones or decorative objects to obtain a desired shade of colors in these crystals wherein optimum treatment cycle (heating time and temperature) is employed. There is a further need to develop a method wherein tone of the imparted colors in colorless gems or crystals can be controlled.